The electrical stimulation of nerves, often afferent nerves, to indirectly affect the stability or performance of a physiological system can provide functional and/or therapeutic outcomes, and has been used for activating target nerves to provide therapeutic relief of pain.
While existing systems and methods can provide remarkable benefits to individuals requiring therapeutic relief, many issues and the need for improvements still remain.
Many techniques, such as non-narcotic analgesics described below, have been developed to treat pain, but all of them are ultimately insufficient.
Psychological strategies, such as biofeedback and psychotherapy, may be used as an adjunct to other therapies but are seldom sufficient, and there are few studies demonstrating efficacy.
Electrical stimulation systems have been used for the relief of pain, but widespread use of available systems is limited.
There exist both external and implantable devices for providing electrical stimulation to activate nerves and/or muscles to provide therapeutic relief of pain. These “neurostimulators” are able to provide treatment and/or therapy to individual portions of the body. The operation of these devices typically includes the use of an electrode placed either on the external surface of the skin and/or a surgically implanted electrode. In most cases, surface electrode (s), cuff-style electrode (s), paddle-style electrode(s), spinal column electrodes, and/or percutaneous lead(s) having one or more electrodes may be used to deliver electrical stimulation to the select portion of the patient's body.
One example of an indication where therapeutic treatment may be provided is for the treatment of pain, such as to provide a therapy to reduce pain in individuals with amputated limbs. Amputation leads to chronic pain in almost all (95%) patients, regardless of how much time had passed since the amputation. The pain can be extremely bothersome to amputees, significantly decrease their quality of life, correlate with increased risk of depression, and negatively affect their inter-personal relationships and their ability to return to work. The present methods of treatment, which are primarily medications, are unsatisfactory in reducing amputation-related pain, have unwanted side effects, offer a low success rate, and often lead to addiction.
Most amputees have two types of pain: residual limb (stump) pain and phantom pain. Approximately 72-85% of amputees have phantom pain and 68-76% of amputees have residual limb (stump) pain. Both stump pain and phantom limb pain are chronic pains experienced after an amputation, and they are easily distinguished by the perceived location of the pain. Stump pain is sensed in the portion of the limb that remains after amputation, and phantom limb pain is perceived in the portion of the limb that has been removed. Typically, amputee patients with severe stump pain also have severe phantom limb pain, but it is recommended that their responses to treatment be measured independently. Stump and phantom pain can be severe and debilitating to a large proportion of persons with amputations, who will unfortunately often progress through a battery of management techniques and procedures without finding relief from their pain.
It has been estimated that 80-95% of 1.7 million persons who currently live with amputations, plus the additional 185,000 persons expected to undergo amputation each year in the United States, suffer or will suffer from stump and/or phantom pain at an annual direct cost of $1.4-2.7 billion and overall associated costs of $13 billion. Severe post-amputation pain often leads to further disability, reduced quality of life, and frequently interferes with the simple activities of daily life more than the amputation itself, and no available therapy is sufficient to manage it.
Many techniques have been developed to treat post-amputation pain, but all of them are ultimately insufficient. One review indicated that none of the then 68 treatments available for post-amputation pain were uniformly successful, and later reviews have found that little has changed; there remains a large need for an effective method of treating stump and phantom pain. Some studies report that as few as 1% of amputees with severe phantom and stump pain receive lasting benefit from any of the available treatments. Presently, most patients are managed with medications, but approximately a third of amputees still report severe (intensity of 7-10 on a scale of 0-10) phantom and stump pain.
Non-narcotic analgesics, such as acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDS), have relatively minor side effects and are commonly used for several types of pain. However, they are general, non-targeted attempts that are rarely sufficient in managing moderate to severe chronic post-amputation pain.
The use of narcotic analgesics, such as N-methyl-D-aspartate (NDMA) antagonists, has shown only minor success with inconsistent results. Narcotics carry the risk of addiction and side effects, such as nausea, confusion, vomiting, hallucinations, drowsiness, dizziness, headache, agitation, and insomnia. Several trials of multiple narcotic agents have failed to show statistically significant improvement in phantom pain.
Physical methods, such as adjusting a prosthesis, may be helpful to reduce post-amputation pain, but generally only if such pain is due to poor prosthetic fit. Other physical treatments, including acupuncture, massage, and percussion or heating/cooling of the stump, have few complications but also have limited data to support their use and have not been well accepted clinically.
Psychological strategies, such as biofeedback and psychotherapy, may be used as an adjunct to other therapies but are seldom sufficient, and there are few studies demonstrating efficacy and these approaches are not specific to stump or phantom pain. Mirror-box therapy has demonstrated mixed results and is not widely used in clinical practice.
Many surgical procedures have been attempted, but few are successful and most are contraindicated for the majority of the amputee patients. Because neuromas may be implicated with stump and phantom pain, there have been many attempts to remove them surgically, but ultimately a new neuroma usually develops each time a nerve is cut and the pain relief only lasts for the time that it takes for a new neuroma to form, usually about three weeks. Furthermore, neuroablative procedures carry the risk of producing deafferentation pain, and any surgical procedure has a greater chance of failure than success. Thus, present medical treatments of stump and phantom pain are inadequate, and most sufferers resort to living with pain that is poorly controlled with medications.
Electrical stimulation systems hold promise for relief of post-amputation pain, but widespread use of available systems is limited.
Transcutaneous electrical nerve stimulation (TENS) has been cleared by the FDA for treatment of pain. TENS systems are external neurostimulation devices that use electrodes placed on the skin surface to activate target nerves below the skin surface. TENS has a low rate of serious complications, but it also has a relatively low (i.e., less than 25%) long-term rate of success.
Application of TENS has been used to treat pain successfully, but it has low long-term patient compliance, because it may cause additional discomfort by generating cutaneous pain signals due to the electrical stimulation being applied through the skin, and the overall system is bulky, cumbersome, and not suited for long-term use.
In addition, several clinical and technical issues associated with surface electrical stimulation have prevented it from becoming a widely accepted treatment method. First, stimulation of cutaneous pain receptors cannot be avoided resulting in stimulation-induced pain that limits patient tolerance and compliance. Second, electrical stimulation is delivered at a relatively high frequency to prevent stimulation-induced pain, which leads to early onset of muscle fatigue in turn preventing patients from properly using their arm. Third, it is difficult to stimulate deep nerves and/or muscles with surface electrodes without stimulating overlying, more superficial nerves and/or muscles resulting in unwanted stimulation. Finally, clinical skill and intensive patient training is required to place surface electrodes reliably on a daily basis and adjust stimulation parameters to provide optimal treatment. The required daily maintenance and adjustment of a surface electrical stimulation system is a major burden on both patient and caregiver.
Spinal cord stimulation (SCS) systems are FDA approved as implantable neurostimulation devices marketed in the United States for treatment of pain. Similar to TENS, when SCS evokes paresthesias that cover the region of pain, it confirms that the location of the electrode and the stimulus intensity should be sufficient to provide pain relief and pain relief can be excellent initially, but maintaining sufficient paresthesia coverage is often a problem as the lead migrates along the spinal canal.
Spinal cord stimulation is limited by the invasive procedure and the decrease in efficacy as the lead migrates. When it can produce paresthesias in the region of pain, spinal cord stimulation is typically successful initially in reducing pain, but over time the paresthesia coverage and pain reduction is often lost as the lead migrates away from its target.
Lead migration is the most common complication for spinal cord stimulators occurring in up to 45-88% of the cases. When the lead migrates, the active contact moves farther from the target fibers and loses the ability to generate paresthesias in the target area. SCS systems attempt to address this problem by using leads with multiple contacts so that as the lead travels, the next contact in line can be selected to be the active contact. Peripheral nerve stimulation may be effective in reducing pain, but it previously required specialized surgeons to place cuff- or paddle-style leads around the nerves in a time consuming procedure.
Immediately following an amputation, all patients experience short-term (postoperative) pain, but it usually resolves within a month as the wound heals. In contrast, a long-term pain often develops and persists in the residual limb, and may be perceived in the phantom limb, after the amputated limb has healed. Residual limb pain and phantom limb pain are thought to have a peripheral and central component, and both components may be mediated by stimulating the peripheral nerves that were transected during amputation.
Neuromas develop when a peripheral nerve is cut and the proximal portion produces new axon growth that forms a tangled mass as it fails to connect with the missing distal portion of the nerve. All amputations produce neuromas and not all neuromas are painful, but neuromas are thought to be a major source of pain after amputation. Neuromas may generate spontaneous activity, and the level of activity in afferent fibers innervating the region of pain has been linked to the level of post-amputation pain.
As previously described, electrical stimulation has been used and shown to be effective in treating amputee pain, but present methods of implementation have practical limitations that prevent widespread use. External systems are too cumbersome, and implanted spinal cord stimulation systems often have problems of lead migration along the spinal canal, resulting in either the need for frequent reprogramming or clinical failure.
Percutaneous, intramuscular electrical stimulation for the treatment of post-stroke shoulder pain has been studied as an alternative to surface electrical stimulation. A feasibility study and a pilot study showed significant reduction in pain and no significant adverse events when using percutaneous, intramuscular electrical stimulation in shoulder muscles.
This form of percutaneous, intramuscular electrical stimulation can be characterized as “motor point” stimulation of muscle. To relieve pain in the target muscle, the percutaneous lead is placed in the muscle that is experiencing the pain near the point where a motor nerve enters the muscle (i.e., the motor point). In “motor point” stimulation of muscle, the muscle experiencing pain is the same muscle in which the lead is placed. In “motor point” stimulation of muscle, the pain is felt and relieved in the area where the lead is located.